In the mechanical processing of the core of the plastic mold, the electric discharge machining method is often used. The electrical discharge machining method is an excellent machining method that can easily perform shape processing even on hard steel materials after quenching. It is also suitable for processing precise shapes and three-dimensional curved surfaces.
However, if you do not fully understand the unique machining characteristics of electrical discharge machining, defects will occur when it is applied to mold parts. Therefore, let us return to the basic electrical discharge machining process.
In electrical discharge machining, electrodes are usually made to face the workpiece (workpiece). After the two sides are applied with different potentials, the electrodes gradually approach in the machining oil. The heat energy generated by the high-temperature heat generated during arc discharge is used to reduce the heat of the workpiece. Part of it melts and quickly decomposes the processing method.
The discharge gap between the electrode and the workpiece is called the discharge gap. The discharge gap is generally about 0.03 to 0.3 mm. Adjust according to the magnitude of the applied current. The applied current is direct current, with positive and negative electrodes. Generally speaking, the electrode side is the positive electrode (anode) and the working side is the negative electrode (cathode). Such processing methods, processability, and electrode consumption are more reasonable.
The electric discharge occurs within a short time of 500,000th of a second to one thousandth of a second within a distance of a few microns, and locally rises to a temperature above the melting point of the steel. Of course, the surface of the workpiece will melt instantly. In addition, the processing oil will instantly evaporate into gas. The gas after the processing oil evaporates blows away the molten part of the workpiece, creating a small depression (crater).
The electrode is moved up and down by the servo motor to restart the next discharge.
Because of the countless crater marks on the surface of the workpiece, there will be fine irregularities and very rough. In addition, in the case of steel materials, the carbon component in the processing oil is immersed in the molten steel material to become martensite with a large amount of carbon (crystal structure during quenching) in many cases, leaving residual stresses and also generating There are countless tiny cracks (microcracks).
An unstable hardened layer several micrometers thick was formed on the surface after electrical discharge machining. However, if the surface after electrical discharge machining is not carefully polished with a grinding wheel or whetstone, and the hardened layer is not removed, the microcracks will gradually increase after the continuous processing of injection molding, and sometimes lead to defective mold parts.
During the electrical discharge machining process, the electrode itself will melt and lose through the electrical discharge. The degree of loss can be controlled to a certain extent according to the current conditions, but it is impossible to not lose at all. Therefore, usually two or more electrodes should be prepared for rough machining and finishing.